What physical property is identical for the isomers of C_5H_12 ?

$\text{The molecular mass. }$
Pentane, ${C}_{5} {H}_{12}$, can generate 3 structural isomers, $\text{n-pentane}$, $\text{isopentane}$, and $\text{neopentane}$, each with distinct chemical and physical properties. The molecular mass is one property that necessarily remains constant for all the isomeric forms.